1. Field of the Invention.
This invention relates to a bar code symbology that can be accurately decoded despite misreads or altered symbols. More particularly this invention relates to a 1-dimensional, fixed format bar code symbology designed to resist the effects of unintended internal spaces and occasional bar location errors that are typical of impact dot-matrix printing, while also offering high security and error checking capability. The symbology combines "delta distance" start and stop patterns, a space-efficient bit-oriented data character structure, and Reed-Solomon redundancy for error detection and correction.
2. Description of the Prior Art.
Installed receipt printers at activities such as lottery outlets may wear mechanically and exhaust their ribbons to the point that any traditional bar code symbols printed by them are unreadable, for two reasons: (1) spaces may appear between adjacent dot rows, that is, within wide "bars," and (2) occasional dot rows are mispositioned by as much as one half of a dot spacing. The conventional bar code symbologies, scanners, and decoders are poorly designed to tolerate these sorts of errors.
There are two basic methods for making a bar code pattern robust against unintended spaces. The first is to build it with only narrow bars separated by spaces 1, 2, and up to "n" dot modules wide, with only the spaces' widths carrying any significance; this is known in the art as a "delta distance" code. The second method is to adopt a fixed-width pattern structure that encourages decoding by time slicing each character into modules, then sampling each module's midpoint for the data bits.
Bar code symbologies generally incorporate start and stop patterns for ease of decoding. Several delta distance symbologies have been developed, from an early IBM code (designed to be printable by line printers) to "BC412" recently adopted for use on semiconductor wafers. These symbologies generally require a quiet zone prior to the beginning of the bar code information. PDF417, a newer two dimensional bar code systems has start and stop codes which are not delta distance, and has relaxed constraints on the quiet zone. However PDF417 does require at least a minimal quiet zone.
Code One is a two-dimensional matrix symbology that can encode the full ASCII 256 character set in addition to four function characters and a pad character. This symbology has ten different versions defined, with 27-50% of the symbol characters allocated to error detection and correction. Some versions do not require quiet zones. Other versions use Reed-Solomon error correction check words.
To the inventor's knowledge, a one dimensional fixed format bar code structure combining "delta distance" start and stop patterns, a space-efficient bit-oriented data character structure, and Reed-Solomon redundancy for error detection and correction has not been realized in the art.